1. Field of the Invention
This invention relates to flat panel displays, more particularly to flat panel displays with scattering shields surrounding phosphor subpixels defining subpixel volumes which substantially reduce the number of scattered electrons that exit from their corresponding subpixel volume and (i) charge display internal structures that have insulating surfaces, (ii) strike non-corresponding phosphor subpixels or (iii) reenter other subpixel volumes.
2. Description of the Related Art
Field emission devices include a faceplate, a backplate and connecting walls around the periphery of the faceplate and backplate forming a sealed vacuum envelope. Generally in field emission devices the envelope is held at vacuum pressure, which in the case of CRT displays is about 1xc3x9710xe2x88x927 torr or less. The interior surface of the faceplate is coated with light emissive elements, such as phosphor or phosphor patterns, which define an xe2x80x9cactive regionxe2x80x9d of the display. Cathodes (field emitters) located adjacent to the backplate, are excited to release electrons that are accelerated toward the phosphor on the faceplate, striking the phosphor, and causing the phosphor to emit light seen by the viewer at the exterior of the faceplate. Emitted electrons from each field emitter are intended to strike only a certain targeted phosphor subpixel. There is generally a one-to-one correspondence between each group of emitters and a phosphor subpixel or a small number of phosphor subpixels for each group of emitters.
Flat panel displays are used in applications where the form-factor of a flat display is required. These applications are typically where there are weight constraints and the space available for installation is limited, such as in aircraft or portable computers.
A certain level of color purity and contrast is needed in CRT displays. Contrast is the comparative difference between dark and bright areas. The higher the contrast, the better. The parameters of resolution, color-purity and contrast in CRT displays depend on the precise communication of selected electron emitters with corresponding phosphor pixels.
High picture brightness, as measured in nits, requires either high power consumption or high phosphor efficiency.
High power consumption in many applications is not acceptable. Efficiency for many phosphors increases as the operating anode voltage increases; the required operating brightness can be achieved with lower power consumption at high voltage. To satisfactorily operate at high anode voltages, e.g., 4 kV or higher, the backplate containing the emitter array must be spatially separated from the faceplate, containing the phosphor pixels, by a distance sufficient to prevent unwanted electrical events between the two. This distance is typically greater than 0.5 mm.
Constrained by faceplate and backplate glass area and thickness, the vacuum envelope is unable to withstand 1 atmosphere or greater external pressure without inclusion of internal supports. If the internal supports are not included then the faceplate and backplate can collapse. In rectangular displays with greater than approximately a 1 inch diagonal, the faceplate and backplate are susceptible to this type of mechanical failure due to their high aspect ratio, which is defined as the larger dimension of the display divided by the thickness of the faceplate or backplate. The use of internal supports in the interior of the field emission device substantially eliminates this mechanical failure.
The faceplates and backplates for a desired flat, light portable display are typically about 1 mm thick. Internal supports, providing support in the interior of the display, may include an edge metallization layer to form an electrical connection between the internal supports and the backplate. However, charge can build up on the internal supports from sources including electrons back-scattered off of the faceplate. See WO patent application Ser. No. 94/18694, filed Feb. 1, 1994; assigned to the same assignee. This application is incorporated herein by reference.
As previously mentioned, there is usually a one-to-one correspondence between each group of field emitters and a phosphor subpixel. High energy electrons from the field emitters may become back scattered from their intended phosphor subpixel and strike another phosphor subpixel, which may be the wrong color. This reduces color purity and contrast. Additionally, these back scattered electrons can strike internal supports causing them to build up charge.
Back scattered electrons pose relatively insignificant problems in conventional CRT""s or low voltage field emission displays. With conventional CRT""s the phosphor is in a field free region. Scattered electrons are collected in the funnel, as illustrated in FIG. 1.
Lower voltage field emission displays, typically less than 1 kV, are not significantly affected by back scattered electrons.
In one low voltage display, all of the electrons are confined to one pixel by switching the neighboring pixels off, as illustrated in FIG. 2.
Higher voltage displays have been carbon coated to attenuate back scattering. However, this method gives less than a 2xc3x97 improvement in contrast. J. J. van Oekel, xe2x80x9cImproving the Contrast of CRTs under Low Ambient Illumination with a Graphite Coatingxe2x80x9d, SID International Symposium, Digest of Technical Papers, 1st Ed., pp. 427-43 (May, 1995).
It would be desirable to provide a high voltage display with scattered electrons that has improved contrast and color purity. It would be further desirable in a high voltage display to reduce the number of scattered electrons that strike non-corresponding phosphor subpixels, or internal insulating and resistive surfaces (e.g., surfaces of internal supports). It would also be desirable to provide, in a high voltage display, a plurality of scattering shields, defining a subpixel volume, to reduce electron escape.
Accordingly, an object of the invention is to provide a flat panel display with improved contrast and color purity.
Another object of the invention is to provide a high voltage flat panel display with a reduction of back scattered electrons that strike the wrong phosphor subpixel.
A further object of the invention is to provide a high voltage flat panel display with a reduction of back scattered electrons that charge up internal insulating or resistive structures.
Yet another object of the invention is to provide a flat panel display with a faceplate interior side that includes a plurality of scattering shields surrounding each phosphor subpixel and defining a subpixel volume.
Another object of the invention is to provide a flat panel display with scattering shields surrounding each phosphor subpixel, defining a subpixel volume. Scattered electrons are confined to the subpixel volume.
These and other objects of the invention are attained in a flat panel display which includes a faceplate with a faceplate interior side, and a backplate with a backplate interior side that is in an opposing relationship to the faceplate interior side. Side walls, positioned between the faceplate and the backplate, form an enclosed sealed envelope of the display. A plurality of phosphor subpixels are positioned at the faceplate interior side. A plurality of field emitters emit electrons which are accelerated to a corresponding phosphor subpixel. A plurality of scattering shields surround each phosphor subpixel. The scattering shields define a subpixel volume. The scattering shields reduce the number of scattered electrons able to escape from the subpixel volume.
The height of the scattering shields surrounding a phosphor subpixel is sufficient to reduce the number of scattered electrons exiting their corresponding subpixel volume and charging internal insulating surfaces in the envelope. Further, the height of the scattering shields is sufficient to reduce the number of scattered electrons from exiting the corresponding subpixel volumes and striking a non-corresponding subpixel.
The height of the scattering shields can be about 20 to 100 xcexcm above phosphor, or above a top surface of an Al layer overlying the phosphor. Further, the height of the scattering shields can be about 20 to 200 xcexcm above the phosphor.
The scattering shields can define a display internal structure that aligns field emitters to corresponding phosphor subpixels. One or more internal supports may be included in the envelope to support the backplate and the faceplate against forces acting in a direction toward the envelope. The scattering shields may be made of a photo patternable material including but not limited to polyimide. Further, the scattering shields may be at least partially formed of a black matrix material.
Inclusion of the scattering shields provides an improvement in contrast and color purity and reduces charging. The scattering shields substantially trap scattered electrons in their corresponding subpixel volumes.